The present invention relates to gamma spectrometric diagraphy system for the determination of the geological parameters of a rock. This sytem more particularly makes it possible to determine the lithology of a rock, i.e. its nature, the nature of the ores and more specifically the heavy elements contained therein, the content of the ores and the density of the rock. The system according to the invention can be used in prospecting for raw materials, such as uranium and certain other metals (lead, iron, etc) or oil.
The measurement of the gamma radiation has long been used in geophysics for determining the characteristics of a rock. The gamma radiation diffused or scattered by a rock is detected by a probe, which converts this radiation into electrical pulses. The analysis of the energy spectrum of these pulses makes it possible to determine certain geological characteristics of the rock.
At present several measuring principles are used and reference is made to some of these hereinafter.
Natural gamma diagraphy involves the probe detecting the gamma radiation emitted by the rock. This method is mainly used for the determination of the lithology and for detecting the natural radioactivity of the rock, particularly for uranium prospecting. In this method, the uranium content is determined either by total gamma counting, or by counting in two energy windows.
In the "universal" gamma-gamma diagraphy method, the intensity of the gamma radiation scattered in the rock and coming from a gamma radiation source contained in the probe is measured. This method is used for determining the density of the rock. Selective gamma-gamma diagraphy is based on the measurement in certain energy windows of the intensity of the gamma radiation scattered by the rock and coming from a gamma radiation source contained in the probe. These methods are based on the counting of gamma photons in one or more energy windows of relatively great width or on the determination of the position of the maximum of the spectrum. They are used for solving certain specific problems, such as determining the content of iron, zinc or lead ores.
In neutron-gamma diagraphy, the secondary gamma radiation emitted by the rock under the effect of a bombardment with neutron from a neutron source contained in the probe is measured. This method makes it possible to detect hydrogen and consequently water and hydrocarbon and in this way indicates the porosity of the rock.
The different aforementioned methods suffer from a number of disadvantages. A first limitation is that each method only makes it possible to determine a single geological parameter and it is consequently necessary to perform a number of measurements with different probes to determine all the geological parameters characterising the analysed rock.
Another disadvantage is that the measurement results relating to one geological parameter can be disturbed by variations in all the geological parameters which are not always known. For example, when determining the uranium content by a total count of the gamma radiation intensity, The result is dependent on the radioactive disequilibrium which can vary from one point to another of a deposit and even along a bore. In the same way, the results of density measurements by the gamma-gamma method are dependent on the lithology, so that the probe has to be calibrated for each particular lithology. Finally, in per se known manner, the determination of the nature of ores (heavy elements) contained in the rock takes place by selective gamma-gamma diagraphy or by neutron-gamma diagraphy, by extracting characteristic peaks of the energy spectrum in the latter case. This extract is difficult, because an intense continuous spectrum due to diffusions or scatterings in the rock and in the bore are superimposed on these peaks and mask them. According to the prior art, this continuous spectrum is not used for determining geological parameters and is instead eliminated so as to only retain the characteristic peaks of the different minerals. The other geological parameters are independently determined by complementary measurements.
A first objective of the invention is to make it possible to determine a maximum number of geological parameters from a single spectrum measurement. It is known that the geological informations are contained not only in the characteristic peaks of a spectrum, but also in the continuous spectrum, of the article entitled "A universal gamma-gamma method for simultaneous determination of rock and ore properties" by J. CHARBUCINSKI, published in Nuclear geophysics, Pergamon Press, 1983, pp 353-361.
The determination of geological parameters by means of a single gamma radiation spectrum has the first advantage of limiting the number of passages of the probes through the drill hole or bore and consequently the duration of the measurements, whilst only a single universal probe is used.
Moreover, simultaneously obtaining information on the different geological parameters makes it possible to improve the precision of the measurements. For example, having knowledge of the lithology makes it possible to choose the appropriate calibration curve for determining the density. The system according to the invention can be used for processing the results of spectrometric gamma-gamma measurements, spectrometric measurements of the natural gamma radiation and for processing the continuous component of the spectrum recorded during neutron-gamma measurements.
In the latter case the nature and content of the ores can be determined in per se known manner by detection of the characteristic peaks. The originality of the invention here is that it takes into account the continuous spectrum in order to determine, without any supplementary measurement, the lithography and density of the mother rock or matrix, whereas in the prior art the continuous spectrum is not analysed and is instead eliminated to permit the analysis of the peaks.
The system according to the invention is based on the exploitation of the photoabsorption phenomenon in the rock. The shape of the spectrum of the gamma radiation scattered in the rock is mainly determined by two physical processes, namely Compton scattering and photoabsorption. Compton scattering is an interaction between a gamma photon and a free electron, or an electron whose bonding energy to the atom is negligible compared with the energy transmitted by the photon. In this interaction, the gamma photon loses part of its energy and undergoes a direction change. The means energy loss decreases with the initial energy decrease of the photon. For this reason, there is an accumulation of low energy photons in the spectrum of the scattered photons. This accumulation is limited by the photoabsorption of the photons, whose energy is comparable to the bonding energy of the electrons in the atom. The effective cross-section for the photoabsorption increases very rapidly when the energy of the photon decreases and is proportional to E.sup.-3.5, in which E is the energy of the photon. The shape of the spectrum and the position of its maximum are dependent on the chemical composition of the rock, or more directly on the ratio of the photoabsorption coefficient to the total Compton mass attenuation coefficient of said rock.
The principle of the invention is to determine the geological parameters by solely exploiting the photoabsorption phenomenon in the analysed rock. To this end, the system according to the invention has means making it possible to extract from the measured spectrum the parameters depending solely on the photoabsorption. This extraction is carried out by correcting the recorded spectrum with the aid of a reference spectrum, which mainly takes account of the Compton scattering.
According to the invention, this standardized spectrum is then compared with models of spectra contained in a library, said comparison consisting of seeking the spectrum model which is closest or the combination of spectrum model closest to the standardized recorded spectrum. On the basis of a single recorded spectrum, this method makes it possible to determine the lithology and/or the nature of the ores contained in the analysed rock.
For determining the lithology for example, the recorded standard spectrum is compared with lithology standard spectrum models. For determining the nature of the ores contained in the rock, the recorded standard spectrum is compared with standard spectrum models of ores.